Suggest a game for my students

I am looking for a relatively short game - 2 to 4 hours - that is free, easy to play, and has an interesting message. I was thinking of using "Gone Home" or "Papers, Please", but they aren't free.

The purpose is for the students to play the game and analyse its effectiveness as a medium of communication. That's why it need to have a message. The quality of the graphics is irrelevant. The key things are free, short, strong message. It would also be good if it involved some thinking skills or other skills, and wasn't just point and click.

All suggestions gratefully received.
 
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